The present invention relates to an optical component suitably applicable in optical communication and others and, more particularly, to an optical fiber grating device in which a long period grating is formed in the core region of optical fiber. The long period grating herein, for example as disclosed in U.S. Pat. No. 5,703,978, converts light of a selected wavelength out of the core mode propagating as confined in the core region, into cladding mode and radiates the cladding mode into the outside of the cladding region, different from short period gratings which reflect light of a selected wavelength.
The optical fiber grating devices with the long period grating in the core region of optical fiber are able to bring about coupling between the core mode of a predetermined wavelength and the cladding mode through periodic perturbations of the grating. Namely, the optical fiber grating devices transfer power of the core mode of the predetermined wavelength to the cladding mode on a wavelength selective basis. Here the core mode is a mode propagating as confined in the core region of optical fiber. On the other hand, the cladding mode is a mode radiated into the cladding region around the core region, without being confined in the core region of optical fiber. Such optical fiber grating devices have been used as optical fiber filters in the fields of optical communications, selectively cutting off core mode of a predetermined wavelength (transmission loss peak wavelength) from among those of a certain wavelength band having propagated in optical fiber.
The cladding mode is a mode taking account of the entire fiber region defined at the interface between the cladding region and an outer layer such as an air layer or a coating layer. Accordingly, a change in the refractive index of the outer layer results in shifting the wavelength of coupling between core mode and cladding mode, i.e., the transmission loss peak wavelength and also changing the transmission loss of the core mode at the transmission loss peak wavelength. Particularly, where the optical fiber is coated with a resin having a refractive index close to that of glass, no cladding mode is formed, so as to result in no transmission loss peak wavelength in the optical fiber grating device. For this reason, the optical fiber grating device had the problem that the coating intended for protection thereof was not applicable.
The optical fiber grating device disclosed in Japanese Patent Application Laid-Open No. 11-326654 is one proposed in order to solve this problem, in which the long period grating is formed in a silica-based single-mode fiber having a refractive index profile of dual shape core (DSC) structure. Here the refractive index profile of DSC structure is of a structure including a first core region with a refractive index n1, a second core region with a refractive index n2, and a cladding region with a refractive index n3 in the order named from the center of the optical axis (where n1 greater than n2 greater than n3). This optical fiber grating device is a device wherein GeO2 is added to both the first core region and the second core region of the optical fiber and they are exposed to spatially intensity-modulated ultraviolet light to form index modulation or a grating across these two regions. In this optical fiber grating device, the core mode of a predetermined wavelength propagating in the first core region is coupled with a higher order mode propagating in both the first core region and the second core region to cut off the core mode of the predetermined wavelength.
The aforementioned Application describes that the preferred refractive index profile in the optical fiber grating device as disclosed in the Application is such that the relative refractive index difference of the first core region is in the range of 0.8% to 1.0%, the relative refractive index difference of the second core region in the range of 0.05% to 0.15%, the radius of the first core region in the range of 3.2 xcexcm to 3.8 xcexcm, and the radius of the second core region in the range of 17 xcexcm to 20 xcexcm.
The inventors investigated the above-stated prior art and found the following problem.
Namely, since this optical fiber grating device includes few higher modes propagating in both the first core region and the second core region, the number of transmission loss peaks is approximately 1 or 2 in the operating wavelength band. Accordingly, freedom of setting the grating period becomes small, when the grating period is set in formation of the grating so as to yield a desired transmission loss peak wavelength in the optical fiber grating device. Since the coupling coefficient is large between the core mode of the predetermined wavelength propagating in the first core region and the higher order mode propagating in both the first core region and the second core region, change is large in the transmission loss and the transmission loss peak wavelength against increase of refractive index due to the exposure to the ultraviolet light and it is thus difficult to achieve control to satisfy both the predetermined transmission loss and the transmission loss peak wavelength.
The present invention has been accomplished in order to solve the above problem and an object of the invention is to provide an optical fiber grating device giving high degrees of freedom in the setting of the grating period and permitting easy control of the transmission loss and the transmission loss peak wavelength.
An optical fiber grating device according to the present invention is an optical fiber grating device in which a long period grating is formed in a core region of an optical fiber comprising a core region having a refractive index n1 and an outside diameter 2a, a first cladding region surrounding the core region and having a refractive index n2 and an outside diameter 2b, and a second cladding region surrounding the first cladding region and having a refractive index n3 and an outside diameter 2c. There is a magnitude relation of n1 greater than n2 greater than n3 among the refractive index n1 of the core region, the refractive index n2 of the first cladding region, and the refractive index n3 of the second cladding region. A relative refractive index difference of the first cladding region to the second cladding region is not less than 0.5%. A thickness (cxe2x88x92b) of the second cladding region with respect to a transmission loss peak wavelength xcex is in a range of not less than xcex nor more than 10xcex.
This optical fiber grating device includes many transmission loss peaks in a predetermined wavelength band (e.g., 1.2 xcexcm to 1.8 xcexcm) and the spacing is small between the transmission loss peaks. The coupling coefficient is small between the core mode and the higher order mode propagating in the first cladding region, so that variation is reduced in the transmission loss and the transmission loss peak wavelength against refractive index change. Further, there is little influence from the outer layer outside the second cladding region, the transmission loss characteristics are stable, and the excellent cutoff effect is achieved in the grating. Accordingly, freedom of setting the grating period becomes large, controlling of the transmission loss and the transmission loss peak wavelength gets easy, and excellent transmission loss characteristics is obtained in this optical fiber grating device.
In the optical fiber grating device according to the present invention, the relative refractive index difference of the first cladding region to the second cladding region is preferably not less than 0.5% nor more than 1.5%. When the second cladding region is doped with the fluorine (F) element, the refractive index n3 of the second cladding region can be made lower than the refractive index n2 of the first cladding region and the relative refractive index difference of the first cladding region to the second cladding region can be increased up to 1.5%.
In the optical fiber grating device according to the present invention, a resin coating having a refractive index n4 (where n4 greater than n2) is preferably provided around the second cladding region. This configuration favorably facilitates guiding of higher order mode generated by mode coupling at the transmission loss peak wavelength to the outside of the optical fiber. This structure is also preferable in terms of protection of the optical fiber grating device.
In the optical fiber grating device according to the present invention, the outside diameter 2b of the first cladding region is preferably not less than 100 xcexcm. This configuration is preferable in that the number of transmission loss peaks is large and also preferable in that the excellent cutoff effect is obtained.
In the optical fiber grating device according to the present invention, a mode field diameter of fundamental mode propagating in the core region is preferably not more than one tenth of the outside diameter 2b of the first cladding region. This configuration is preferable in that the number of transmission loss peaks is large and also preferable in that the excellent cutoff effect is obtained.
The present invention can be better understood by the detailed description and the accompanying drawings which will follow. It is noted that these are to be considered simply illustrative of the invention, but are not to be considered restrictive to the present invention.